Project Leader, Organisation
Associate Professor Jennifer Firn, Queensland University of Technology
The Threatened Species Committee consider Austopuccinia psidii (myrtle rust, MR) as a serious threat to biodiversity due to impacts on ecologically, culturally, socially and economically important plant species from the Myrtaceae family. Over 350 species in Australia are known as susceptible. MR is killing off the understorey of unique forest ecosystems across eastern and northern Australia; and therefore impacting entire ecosystems. This study was to understand how this loss of understorey species impacts on ecosystem function and whether there are specific plant traits that can be used to predict susceptibility of the host.
Objectives and impact
This project aimed to develop an understanding of the influence of plant traits and site characteristics in the incidence of MR infection and severity of impact on individual species and plant communities. Quantifying these impacts will support the development of focussed species conservation processes and identification of plant communities and ecosystems at greatest risk of long term impacts.
Using sites already identified by Carnegie (2016) and Pegg (2018) we aimed:
a) To determine the mortality of five locally abundant populations of Myrtaceae species over time – Acmena smithii, Archirhodomyrtus beckleri, Decaspermum humile, Gossia hillii and Rhodamnia maideniana in response to repeated infection by A. psidii at the Tallebudgera site in SEQ.
b) To assess the effect of tree mortality/death on canopy transparency by quantifying canopy gap fraction in plots that show evidence of A. psidii induced tree mortality and those without evident mortality.
c) To assess the effect of mid-story tree mortality on the understory vegetation % cover (both native and non-native vegetation), seedling composition and soil nutrients.
d) Finally, we tested whether there was a detectable shift in functional traits of two of the five Myrtaceae species as a result of infection by A. psidii. We used the SLA trait because it has been found to be a surrogate measure of how plants allocate energy and biomass to their leaves. We compared SLA from the TRY database (control) and SLA from the field (treatment) where trees have been infected by A. psidii.
We found plots with more light gaps due to A psidii-induced tree mortality tended to be positively correlated with increased cover of non-native species in the understory (e.g., Lantana camara and Ochna serrulate).
After just four years of studying the site, we found significant changes to light conditions and forest understories as well as a rapidly increasing mortality rates for species that originally showed little mortality. One species that was highly impacted, Archirhodomyrtus beckleri, showed a significant shift in specific leaf area (SLA) between infected populations and uninfected populations, while a less impacted species, Acmena smithii did not.
Specific leaf area is commonly used as an indicator of a plant’s growth strategy. Although only for two species and needing more research, the changes in SLA suggest that the species with a higher mortality rate has a higher SLA (and therefore a resource acquisition specialist) when not infected than a species that shows little mortality. Relative values of SLA and the plasticity of SLA may be surrogate measure for susceptibility to A. psidii, but further research is needed. These early changes in canopy gaps, species composition and vegetation cover will likely become more pronounced over time.
Follow-up studies are highly recommended to document the pathogen impacts on populations and community dynamics so that timelines of impact can be better understood, and interventions optimised.
The Final Report can be downloaded here. A journal paper has been submitted for publication.